a) Field of the Invention
The present invention concerns a process for assembling LPB batteries. More particularly, the invention relates to the lamination of an electrolyte to a positive electrode under conditions enabling an excellent adhesion and preventing the formation of wrinkles at the outlet of the laminating device.
b) Description of Prior Art
The lithium/polymer electrolyte batteries (hereinafter designated LPB) are manufactured by superposing three main types of films: a film of a positive electrode containing an electrochemically active material, for example vanadium oxide, a film of an electrolyte consisting of a polymer and a lithium salt, and a film of metallic lithium. Each of these films is between 15 and 50 .mu.m thick, for a total thickness of the elementary film of battery of 100 to 150 .mu.m. About thirty meters of this elementary film, 15 cm wide, are typically required to give a battery of 100 Wh.
One of the main procedures for manufacturing LPB batteries consists in separately preparing the positive electrode and the electrolyte by solvent coating or by melt extrusion. The positive electrode is thereafter laminated to the electrolyte, then the lithium negative electrode is applied on the second face of the electrolyte. The positive electrode is coated on a metallic foil (for example aluminum) or on a metallized plastic film which serves as current collector, and which is left in the battery after lamination. The electrolyte is coated on a plastic substrate (for example a film of polypropylene), which is eliminated after lamination to the positive electrode and before applying the lithium electrode.
If the lamination is carried out directly with the laminating rolls kept at room temperature, the adhesion between the electrolyte and the positive electrode is not very good and the electrochemical impedance of the interface is high. If the laminating rolls are heated and the temperature of the electrolyte gets higher than its softening temperature (60.degree.-80.degree. C.) during lamination, the adhesion and the electrochemical interface become excellent, but wrinkles are often formed at the outlet of the laminating rolls, when the films return to room temperature, because of the differences in thermal expansion and elongation between the electrolyte and the substrate of the electrolyte and of the electrode. These wrinkles may cause short-circuits in the final battery, and reduce its energy density.